Dunsmuir Ridge and the Irvingtonian gravels

Just northwest of Lake Chabot are some tiny areas mapped as “Irvington Gravels,” high above the Sheffield Village neighborhood in the Dunsmuir Ridge Open Space. They caught my eye because Irvington (part of present-day Fremont) is the site of a famous set of Ice Age fossils, from which the Irvingtonian age of North American land mammals was established. Yesterday I checked the area out, in case there were some sabertooth-cat fangs lying around. This entry has a lot of photos.

You get there starting at the access at the end of Covington Road, a dirt fire road that goes straight up a steep hillside. The Hayward fault crosses the road partway up, at a little level spot at the edge of the woods. To the west of the fault, the rocks are mapped as San Leandro gabbro (Jurassic rocks of the Coast Range Ophiolite), but it’s really hard to tell:

Across the fault the rocks change to Late Jurassic volcanic rocks of the Coast Range Ophiolite, the same stuff exposed in the big Leona quarry:

Higher up are three small terraces where the gravel is mapped. This is looking south from the northernmost one:

It looks like a hopeless task to find rocks here. Luckily for me, the fire roads have recently been graded, so there was a window into the substrate. As I approached the terraces, the roadbed started to display river cobbles, quite unexpected in this setting:

I made a point of crossing the grassy slope to the other two terraces, looking for stones the whole way. Nada. From the southernmost terrace, here’s the view north. Click on the picture for a stereopair:

There’s a house on a knoll at the same height as the terraces. The upper part of the Knowland Park Zoo land also lines up with the terraces. No gravel is mapped at either place, but there might be some.

Now the cobbles in the roadbed start to look interesting:

Above is another, higher terrace. It’s over 500 feet above the starting point and a bit of a trudge.

Just below it are scattered outcrops of the volcanic bedrock:

The roadbeds on the upper terrace also have interesting cobbles. I took a few home to clean up and photograph. Remind me to bring them back on my next visit.

Russ Graymer, who prepared the Oakland geologic map, describes the suite of cobbles thus: “Cobbles . . . consist of about 60 percent micaceous sandstone, 35 percent metamorphic and volcanic rocks and chert probably derived from the Franciscan complex, and 5 percent black laminated chert and cherty shale derived from the Claremont Formation.” He holds that these little terraces started out near Fremont and were carried here by the Hayward fault. They started out at a much lower elevation too, I would think; just a sign that fault movements are not straightforward.

2 Responses to “Dunsmuir Ridge and the Irvingtonian gravels”

Yes, I found those same rounded gravels (in a sand matrix) while I was trenching on that ridgeline a few years ago. Some of the rocks are high-pressure metamorphics.
Those gravels appear to be a fossil of a previous drainage regime, that existed prior to the uplift of the East Bay Hills. How else do you ger this classic level-ground alluvium (looks just like the Niles cone deposits), way up on the hillside ? The creek that deposited those sands and gravels surely did not flow down the face of the current slopes!

There have been several reports made on the geology of Dunsmuir Ridge. Trenching was done in the 80’s when a developer wanted to build several hundred units on the site. Do y’all have access to those reports? Where did your map come from? I’m very interested in the botany and zoology of Dunsmuir Ridge, and how it relates to the geology.